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During the Second Great War, Abathur attempted to improve the evolutions of the Zerg Swarm, and sought out various opportunities for this, including unique adaptations of feral zerg, wild animals with useful biological traits, and abandoned zerg breeds that could be reclaimed for the Swarm. However, due to conflicts between the potential mutations he discovered, only one could be permanently assimilated into the Swarm, while the other would be discarded. Sarah Kerrigan ordered Abathur to make limited groups of either mutation for her to field test them in battle, then and made the personal decision of which to keep.[1]

Individual evolution missions are unlocked by completing specific missions in the main campaign. They focus prominently on a specific breed of zerg, and each mission is divided in two parts, each part allowing the player to utilize a specialty strain of the breed to accomplish their objective. Upon completing the mission the player will be given the choice to permanently upgrade the base strain to one of the two featured strains for the rest of the campaign. In this mission however is that all the mutation traits of each strain are removed and some of Sarah Kerrigan abilities in tier 3 and 5 can be selected to aid your units.

Brood lord strain: Can morph into a brood lord, which besieges ground targets from long range with spawned broodlings.

Viper strain: Can morph into a viper, which can pull distant units close, slow enemy ground units and prevent them from attacking for several seconds, or drain life from allied units to restore its own energy. Also enables air-to-air combat.